Wideband Amplifier Circuits

This page discusses many types of wideband amplifier circuits. These amplifiers are utilized in a variety of high frequency applications in electronics. We'll talk about how they operate and what makes them unique for specific RF circuit designs.

A wideband amplifier is a circuit that boosts electronic signals over a wide range of frequencies, making it useful for various applications where signals vary in frequency.

Wideband Power Amplifier

This wideband power amplifier doesn't generate a substantial amount of power; it typically operates in the milliwatt range.

It employs a configuration similar to the common-base/common-collector amplifier but offers power gain, capable of delivering an output ranging from 10 to 180 milliwatts, depending on the power supply voltage.

The common collector stage provides current gain, facilitating power transfer to the load. To achieve the required bandwidth, both stages must operate at relatively high quiescent current levels.

The common-base stage compensates for the input capacitance of the common collector stage, and the inclusion of an RFC (Radio-Frequency Choke) in the emitter of the output device enhances high-frequency response, expanding the amplifier's bandwidth.

R1 can be adjusted to match the input impedance with the source driving impedance.

If needed, two of these amplifiers can be cascaded to produce approximately half a watt output without significant degradation in overall bandwidth.

Adjusting the 1uF capacitors can alter the lower cutoff frequency.

Specifications

Midband power gain ranges from 13 to 15 dB, and the power output is approximately 10 milliwatts with a 5V supply or 180 milliwatts with a 20V supply.

The bandwidth spans approximately 50 kHz to 15 MHz. The input capacitor should be a 50V metallized poly or monobloc type with low inductance.

Wideband Amplifier with DC Feedback

For achieving high and stable gain, a wideband amplifier necessitates multiple stages with various DC feedback paths.

In this configuration, two left-hand and two right-hand transistors form common-emitter/common-collector pairs.

The common collector devices serve as a high-impedance load for the preceding transistor and a low source impedance for the following stage, minimizing internal capacitive feedback.

The circuit employs a CA3018 transistor array IC, and the lower cutoff frequency depends on the capacitor values. It's recommended to use low self-inductance metallized poly capacitors with low voltage ratings.

Specifications

The midband gain is approximately 50 dB, and the bandwidth (-3 dB) extends from 1 kHz to 30 MHz. The maximum input signal should be around 4 mV RMS.

Wideband Cascade Amplifier

The primary characteristic of the wideband cascade amplifier configuration is its ability to isolate the input from the output, resulting in excellent stability regardless of load reactance.

The lower transistor functions as a common-emitter amplifier, while the upper one operates in grounded-base mode.

This setup employs two transistors from a transistor array IC (CA3046), each with an hFE of 110 and a cutoff frequency (f) of 450 MHz.

The upper cutoff frequency is determined by R1, and increasing capacitor values can reduce lower cutoff frequencies. Alternatively, discrete transistors such as 2N706, 2N2369, 2N3607, MPB3646, or 2N5769 can be used.

Specifications

The midband gain (loaded) is approximately 32 dB, and the bandwidth (-3 dB) spans approximately 5 kHz to 4 MHz. It's crucial to ground pin 13 of CA3046 for proper operation.

For the input capacitor, consider using 50V metallized poly types or tag tantalum capacitors (with attention to polarity).

Common Base, Common Collector Wideband Amplifier

This amplifier configuration offers the advantage of low input and output impedances, typically around 100 Ohms.

It utilizes two devices from a transistor array IC. For optimal results, use dipped tantalum capacitors and keep lead lengths short.

You can adjust the input impedance to match the source impedance by varying R1, which, in turn, modifies the emitter current of the common base stage.

While this wideband amplifier configuration provides excellent linearity, its gain is not exceptionally high.

Specifications

The midband gain is approximately 17 dB, and the bandwidth (-3 dB) ranges from approximately 150 Hz to 3.5 MHz.

The maximum input at a 5V supply should be around 40 mV RMS, and it's essential to ground pin 13 of CA3046 for proper operation.

Wideband Video Amplifier Circuit

This circuit is an excellent choice for a wideband video amplifier. Its design works with a compound series feedback configuration, which offers several advantages such as high input impedance, low capacitance, and consistent gain across a broad frequency spectrum.

In order to get optimal performance, it may be important to use prefect RF construction practices, according to which the component leads must be kept short and ensure proper bypassing of the power supply.

For effective bypassing, you may want to use both a 100n ceramic capacitor and a 1u tantalum capacitor in parallel.

Please note that both the input and output signals remain in phase in this circuit.

In case you want to adjust the gain, this can be accomplished by varying the value of the 10k feedback resistor.

Wideband RF Amplifier Circuit

This wideband RF amplifier features a bandwidth ranging from approximately 100 kHz to 4 MHz and exhibits high gain and stability.

JFET common-source amplifiers are used in the input stage, and Q1 is then directly connected to Q2, a grounded base stage.

A low impedance output is then produced by an emitter follower, Q3.

The grounded base second stage maintains strong stability by offering gain and good input-output separation.

These tantalum-type polarized capacitors are all available. Ceramic should be used for the supply rail bypass capacitor.

The input impedance will depend on the magnitude of R1. This can be set to a small value for sources with low impedance requirements or an increased value, up to a few hundred kilohms, for sources with high impedance requirements.

Similarly, R8 may be chosen to adjust the output impedance. It normally ranges between 39 and 560 Ohms.

R8 might not be included at all if the output connects with a medium to high impedance source.

R3 is tuned to get around 10.5 volts at Q1's drain while the amplifier is initially set up, and it may need to be tweaked a little more for optimal performance afterwards.

Once R3 has been tuned to its ideal value, the trimpot could be replaced by a fixed resistor of an appropriate value. For Q2 and Q3, a large range of transistors are possible.

Wideband Crystal Oscillator Circuit

The following is an universal wideband crystal oscillator with an integrated buffer for parallel resonant crystals.

The source-follower Q2, which has a high input impedance and places the least amount of strain on the oscillator stage, receives feedback from the crystal via Q1, whose output is only weakly linked to it through the 10pF capacitor.

Q3 and Q2 are connected directly. Common-emitter amplifier design, with output obtained from the collector through a 10nF capacitor.

When compared to Q1, which is a straightforward periodic oscillator, the Q2-Q3 buffer offers a wideband amplifier with modest gain.

Any source between roughly 7 V and 15 V can be used to power it.
Use of a regulated supply will result in the highest level of stability and minimal output noise.

Wideband Buffer Circuit

The following design works well as a wideband buffer for RF oscillators ranging in frequency from 100 kHz to 20 or 30 MHz.

It displays a high input impedance and a relatively low output impedance, has a tiny amount of gain, and is extremely stable.

It also works well as an impedance step-down circuit.

A JFET called Q1 is used as a source-follower. With its output coming from the collector, Q2 is directly linked to it and used as a common-emitter amplifier.

For the highest RF efficiency, ceramic capacitors are always selected because of their small self-inductance.

Wideband TV Signal Booster Circuit

A wideband, low noise VHF amp is shown in the following figure to improve TV or scanner reception. It is easy to construct and presents no problems.

There is no optimizing needed.

The signals coming in are coupled to the base of transistor Q1, a BFY90, using a typical 75 to 300 Ohm TV antenna balun. R1-R2 produce the base bias, which is then bypassed by C1 and the 300 Ohm winding of T1.

R4 offers some emitter degeneration [negative feedback] for Q1. Similar to T1, another balun links signals to the output connection.

The output transformer's 300 Ohm winding serves as the source of the collector voltage for Q1, which is then bypassed by C2 and decoupled by R3.